Songbirds and parrots are among the few animal models for human speech in which it is possible to experimentally study the basic physiological and acoustic problems involved in producing complex, learned vocalizations. The usefulness of these birds as a model for peripheral mechanisms of speech production is limited however by the fact that, although the supralaryngeal vocal tract plays a preeminent role in the articulation of speech, it is one of least well understood components of sound production in birds. This research aims to fill that gap by addressing the role and function of the suprasyringeal vocal tract in birdsong. The acoustic properties of the avian trachea and oropharyngeal cavity will be experimentally determined. Vocal tract motor patterns that may modulate sound by varying the length or shape of the vocal tract during song, thus altering its resonant or filter properties will be monitored and quantified. The acoustic role of tracheal, laryngeal, glottal, lingual, beak and oropharyngeal movements will be measured. The location and the nature of tunable vocal tract filters that may track the fundamental frequency of vocalizations, vary the formant structure of broadband sounds or suppress selected harmonic components will be determined. A two chamber model of English vowel production by parrots will be tested. The ontogenetic development of these vocal tract motor programs in relation to other aspects of song learning will be investigated in juvenile birds. The location of motor neurons controlling these motor patterns and their premotor inputs will be determined and the central pathways linking motor control of the vocal tract to the song control circuitry in the brain will be mapped, as will feedback pathways from the lungs and syrinx. The relative roles of auditory and somatosensory feedback in providing closed-loop, real-time modulation of vocal tract motor programs that may compensate for unpredictable peripheral perturbations will also be examined. The knowledge gained from these experiments will significantly improve the understanding of song production and the usefulness of birdsong for studying fundamental mechanisms of vocal control that also apply to speech, including the motor basis of important speech fluency disorders such as stuttering.